REGRESSION: [Mac] Intermittent crashes in AXObjectCache::notificationPostTimerFired
https://bugs.webkit.org/show_bug.cgi?id=106106

Reviewed by Eric Seidel.

The crash was cause by notification post timer firing after the object destruction had taken place.
Clear the timer so that this won't happen. Also make sure the document doesn't go away while we're
going through m_notificationsToPost.

No new tests since crashes have been caught by various layout tests, and we can't easily
create a deterministic layout test for this particular crash.

* accessibility/AXObjectCache.cpp:
(WebCore::AXObjectCache::~AXObjectCache):
(WebCore::AXObjectCache::notificationPostTimerFired):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138916 268f45cc-cd09-0410-ab3c-d52691b4dbfc